When a container made from a thermoplastic resin is subjected to a process color printing on its outer surface, it is generally required to provide a part of the barrel portion of the container with a flat portion to which the printing is applied. Thus, it has only been possible to print, merely a simple stretching pattern on the outer surface of the container.
Often a heat-shrinkable film or sheet is used as a packing material for the container. However, when the film or sheet is subjected to printing and is heat-shrunk onto the container, irregular strains are created on the film or sheet. Thus, merely a simple stretching pattern which permits a slight shift of the position of the stretching pattern can be printed on the film or sheet.
Further, in recent years, many microwavable food containers have been developed, but none of the containers have a printed indication on the outer surface thereof. Also, during cooking of food stuffs in an microwave oven the container is heated to a high temperature due to heated food stuffs, and thus cannot be held with a bare hand, and therefore is inconvenient to be taken out from the microwave oven.
A heat-insulating container which is able to have a high-grade printing on the outer surface thereof has been desired which can list the contents of the food stuffs contained in the container and thus create added values to the container.
In the past, a heat-insulating container was made by adhering a heat-insulating sheet made from a foam sheet was adhered to the outer surface of the container.
The foam sheet has applied to the container, by the following method. The foam sheet is continuously extruded from an extruder while it is stretched, in a direction perpendicular to the direction of the extrusion to produce a sheet. The sheet is subjected to a printing process and then is cut along the direction of the extrusion to produce a long belt. Then, the longitudinal side edges of the belt parallel to the extrusion direction are continuously adhered to each other to produce a cylinder. The cylinder is cut to a desired length to produce a plurality of smaller cylinders. The container is covered with the cylinder and heated, and thus the cylinder is heat-shrunk and wrapped on the container. This method, however, has the following disadvantages.
As a result of an investigation by the inventors, it was found that a large percent of such containers have defects where the edges of the sheet were adhered to form a cylinder. One of the reasons causing the great percent defects may be the fact that since the belt is made round so that the longitudinal direction of the belt may conform with the central axis of the cylinder and the both side edges of the belt are adhered to produce the cylinder, the cylindrical belt will be subjected to a great restoring force tending to deform the belt from the cylindrical shape to the flat shape. However, the above reason is not enough to explain the large percent of defective containers. This is also apparent from the fact that, as will be described later in Comparison Example 1, even when the belt was cut out that the longitudinal direction of the belt is perpendicular to the extrusion direction to produce a rectangular belt and the both side edges of the belt parallel to the extrusion direction were adhered with each other to produce the cylinder, the defective adhesion was easily caused.
A process for cutting the cylinder and putting it on the container and heat-shrinking the cylinder is preferably performed on an on-line system connected with a process for making the cylinder. Most preferably, the process for cutting the cylinder and putting it on the container and heat-shrinking the cylinder is performed concurrently with the cutting of the foam sheet to the belt and the adhesion of the both side edges thereof. In this case, however, the defective adhesion results in a defective heat insulating container. In order to avoid this disadvantage, the following method without using the on-line system may come to mind. A long cylinder in which the defective portions have been eliminated is first wound on a reel; then re-wound from the reel, cut, and then placed on the container and heat-shrunk onto the container. In this method, however, a fold or crease is formed on the cylinder during the winding operation, and this fold can not be removed even after the heat-shrinkage of the cylinder on the container. Thus the appearance of the container will be extremely impaired, and thus the value of the container as an article of commerce will be decreased.